Field of the Invention
The invention relates to a system and a method for providing sound to at least one user, wherein audio signals from an audio signal source, such as a microphone for capturing a speaker's voice, are transmitted via a wireless link to a receiver unit, such as an audio receiver for a hearing aid, from where the audio signals are supplied to means for stimulating the hearing of the user, such as a hearing aid loudspeaker.
Description of Related Art
Typically, wireless microphones are used by teachers teaching hearing impaired persons in a classroom (wherein the audio signals captured by the wireless microphone of the teacher are transmitted to a plurality of receiver units worn by the hearing impaired persons listening to the teacher) or in cases where several persons are speaking to a hearing impaired person (for example, in a professional meeting, wherein each speaker is provided with a wireless microphone and with the receiver units of the hearing impaired person receiving audio signals from all wireless microphones). Another example is audio tour guiding, wherein the guide uses a wireless microphone.
Another typical application of wireless audio systems is the case in which the transmission unit is designed as an assistive listening device. In this case, the transmission unit may include a wireless microphone for capturing ambient sound, in particular from a speaker close to the user, and/or a gateway to an external audio device, such as a mobile phone; here the transmission unit usually only serves to supply wireless audio signals to the receiver unit(s) worn by the user.
The wireless audio link often is an FM (frequency modulation) radio link operating in the 200 MHz frequency band. Examples for analog wireless FM systems, particularly suited for school applications, are described in European Patent Application EP 1 864 320 A1 and corresponding International Patent Application Nos. WO 2006/104634 A2 and WO 2008/138365 A1.
In recent systems the analog FM transmission technology is replaced by employing digital modulation techniques for audio signal transmission, most of them working on other frequency bands than the former 200 MHz band.
U.S. Pat. No. 8,019,386 B2 relates to a hearing assistance system comprising a plurality of wireless microphones worn by different speakers and a receiver unit worn at a loop around a listener's neck, with the sound being generated by a headphone connected to the receiver unit, wherein the audio signals are transmitted from the microphones to the receiver unit by using spread spectrum digital signals. The receiver unit controls the transmission of data, and it also controls the pre-amplification gain level applied in each transmission unit by sending respective control signals via the wireless link.
International Patent Application WO 2008/098590 A1 relates to a hearing assistance system comprising a transmission unit having at least two spaced apart microphones, wherein a separate audio signal channel is dedicated to each microphone, and wherein at least one of the two receiver units worn by the user at the two ears is able to receive both channels and to perform audio signal processing at ear level, such as acoustic beam forming, by taking into account both channels.
International Patent Application WO 2010/078435 A1 relates to a communication system comprising a plurality of transmission units comprising a microphone for capturing the respective speaker's voice and transmitting audio signal data packets to a receiver unit which may be connected to an earphone or to a hearing aid via a plug jack. The transmission units and the receiver unit form a wireless network using a pseudo random sequence frequency hopping scheme and having a master-slave architecture, wherein certain slots in each frame are individually attributed to each of the transmission units, so that each transmission unit is allowed to transmit audio signals in its dedicated slots and receive audio signals transmitted in the remaining slots. Synchronization information data may be transmitted by the master in a certain slot of the frame. Each audio data packet is redundantly transmitted three times in three dedicated slots, with the receiver unit only listening until a correct copy of the audio data packet has been received, so that, when already the first copy is correctly received, the receiver unit would not listen to the redundant copies. Audio signals are encoded by using sub-band ADPCM (Adaptive Differential Pulse Code Modulation), and the packets may be compressed from 16 bits to 4 bits using a G.722 encoder.
International Patent Application WO 99/16050 A1 relates to a scalable and embedded audio codec to be used for internet multimedia applications, wherein a single audio stream is provided for a plurality of devices which may have different sampling rates and/or bit rates. Lower bit rate output bit streams are embedded in higher bit rate bit streams in a manner that low quality audio devices may decode only part of the bit stream, while high quality audio devices may decode the full bit stream. The audio information corresponding to the lowest bit rate application may be inserted in a first priority packet, while secondary information may be inserted in second and third priority packets, so that devices operating only at the lowest bit rate can automatically separate the first priority packets from the remainder of the bit stream and use only these packets for signal reconstruction.
U.S. Pat. No. 5,570,363 relates to a personal computer based conferencing system using a scalable audio codec which provides for a single output audio stream which can be decoded by audio devices having different bandwidths and bit rates. Different data packets are produced for different devices, wherein the packets for higher quality audio devices include additional parts including the surplus of audio information.
U.S. Pat. No. 7,272,556 B1 relates to an audio codec providing compatibility over a range of communication devices operating at different sampling frequencies or bit rates, wherein the input signal is divided in different portions, at least one of which carries information sufficient to provide intelligible reconstruction of the input signal, and wherein separate information about other portions of the signal is encoded in an embedded manner, so that a smooth transition can be achieved from low bit rate to high bit rate applications. Thereby communication devices operating at different sampling rates or bit rates can extract corresponding information from the output bit stream. A similar audio codec is described in US 2008/0052068 A1.
European Patent Application EP 2 129 170 A1 relates to a system for wireless audio signal transmission from a TV-set to a hearing aid, wherein a G.722 audio codec is used.
Receiver devices for high fidelity audio reception, which support high sampling rates and thus offer large audio bandwidths as well as high resolution, typically require a relatively large power source (battery), so that the achievable degree of miniaturization is limited. On the other hand, receiver devices for speech quality audio reception, which support moderate sampling rates and thus offer a reduced audio bandwidth as well as lower resolution, can be designed for relatively low power consumption, so that a relatively high degree of miniaturization can be achieved.
In order to communicate with such different types of receiver devices, the transmission devices have to adapt their encoding scheme to the specific requirements of the receiver devices. Such adaptation of the audio quality to the requirements of a receiver device can be achieved, for example, by employing a sub-band ADPCM codec, such as the G.722 standard. This codec is particularly suited for low complexity, battery powered devices, since the computational requirements for encoding and decoding are reasonable. In addition, the delay introduced by this codec is low, which is particularly interesting for applications like wireless microphones, where lip synchronicity has to be guaranteed, as well as TEM (In-Ear-Monitoring) systems.